This invention relates to ABS-type polymers and more particularly to an improved process for preparing ABS-type polymers having excellent physical properties.
Over the past few years acrylonitrile-butadiene-styrene terpolymer, more commonly known as ABS polymers, have taken over a large share of the market formerly held by other impact polymeric materials such as impact polystyrene. The constantly increasing demand for these polymers is due to the excellent mechanical and chemical properties that they possess; for instance, ABS plastics have greater toughness and impact strength than impact polystyrenes containing the same amounts of rubber and they also possess excellent chemical solvent resistance.
In spite of the excellent mechanical and chemical properties of ABS polymers their growth in the plastics market has been somewhat hindered due to their high cost and poor visual properties. Both of these disadvantages are caused to some extent by the method of commercial production of ABS polymers. Today, by far the greatest amount of ABS polymer on the market is produced by conventional emulsion polymerization techniques. Unfortunately, the prolonged polymerization periods and multi-step post polymerization necessary in emulsion type processes often render products made by this type of process economically unsuitable for many users. In addition, residual emulsifier and other agents still present in the polymer due to incomplete purification cause the polymer to have poor color. For these reasons more efficient and economical processes for producing ABS polymers are constantly being sought.
Recently, a patent has issued to De Bell et al. (U.S. 3,370,105) which discloses a process of preparing rubber-containing impact type polymers such as ABS by the suspension method. The unique feature of this patented process is that the rubbery material onto which the styrene and acrylonitrile are grafted is originally in the form of latex but the latex is coagulated prior to the initiation of the polymerization. Products prepared by this process, although representing an improvement over some prior art products, still leave a lot to be desired from the standpoint of appearance of physical properties.
In another recently issued U.S. Pat. No. 3,436,440, Abe et al disclose an emulsion-suspension process involving the graft polymerization of a mxture of styrene and acrylonitrile onto a butadiene rubbery polymer in latex form. In this process the rubber latex, monomers, a suspending agent, a water-soluble catalyst and an oil-soluble catalyst are all introduced into a reactor; the contents are heated to polymerization temperature and the reaction proceeds in the emulsion state. After the polymerization has proceeded to a certain point the emulsion coagulates and forms a stable suspension. The polymerization is then completed in suspension. The product prepared by this process has very good mechanical properties, however, it may have an undesirable color. This patent also discloses a process in which an oil-soluble catalyst is used alone to produce ABS polymers. Products prepared according to this disclosure have very poor physical properties.
A process has now been discovered for producing ABS polymers having excellent mechanical and visual properties. This process also presents the desirable feature of having a very short polymerization cycle, such as in the range of 4-7 hours, thus the efficiency of the process of the invention is greater than that of comparable prior art processes. Accordingly, it is an object of the invention to present a process for preparing ABS polymers having improved physical properties. It is a second object of the invention to present a short time-cycle process for preparing ABS-type polymers. These and other objects of the invention will become apparent as the description proceeds.